I'd say the 10% blanket discount would be a good idea, then people could get anything they want. The problem with having an offer on just the DVDs, such as the buy 2 get one free that you suggested is that a lot of people might have them all, or might have all of the ones that they want.
Cheers,
Malcolm
'England Expects That Every Man Will Do His Duty' - Vice-Admiral Horatio Lord Nelson - Battle of Trafalgar - 21st October 1805